1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding sheets one by one to an image forming system such as a laser beam printer, copying machine and the like.
2. Related Background Art
An example of a conventional image forming system such as a laser beam printer and a sheet feeding apparatus is shown in FIG. 26.
In FIG. 26, a laser beam printer (image forming system) includes a body frame 302, a pivotable printer front plate 305 pivotally mounted on the body frame via a hinge shaft 303. A process cartridge 306 including a photosensitive drum 307, developing device 309 and the like is removably contained within the body frame 302, and a laser beam L emitted from a laser beam scanner 310 is illuminated onto the photosensitive drum 307 to form a latent image thereon, which latent image is developed by the developing device 309 to obtain a toner image.
A sheet supply tray 312 holding sheets P.sub.1 thereon is removably attached to the printer front plate 305, and the sheets P.sub.1 are separated one by one by means of a sheet supply roller 381 and a separating pad 382 and are fed to a pair of regist rollers 317. The sheet P.sub.1 fed from the regist rollers 317 at a predetermined timing is brought to the photosensitive drum 307, where the toner image is transferred onto the sheet. Thereafter, the sheet is fed to a fixing device 323, where the image transferred to the sheet is fixed to the sheet, and then the sheet is ejected onto an ejector tray 326 by means of a set of ejector rollers 325.
An intermediate plate 383 supporting leading end portions of the sheets P.sub.1 in the sheet supply tray 312 is biased upwardly by means of a spring 385, as shown in FIG. 27. FIGS. 28 and 29 show a mechanism for urgingly engaging or disengaging the sheet P.sub.1 with respect to the sheet supply roller 381. In FIGS. 28 and 29, a drive ring 393 receives a driving force from the body, and the driving force is transmitted to a drive shaft 380 integral with the sheet supply roller 381 via a control ring 390.
As shown in FIG. 28, in a condition that the drive shaft 380 is locked by a pawl 391a of a solenoid 391, the driving force from the drive ring 393 is not transmitted to the drive shaft 380, with the result that the sheet supply roller 381 remains stationary. A support arm 387 formed integrally with the intermediate plate 383 is pivotally mounted on a support shaft 386, and a roller 387a disposed on a free end of the support arm 387 is urged against a largest diameter portion of a pressure cam 389 fixed to the drive shaft 380.
When the solenoid 391 is activated by an electric circuit 392, the pawl 391a is disengaged from the control ring 390, with the result that the driving force of the drive ring 393 is transmitted to the drive shaft 380. When the sheet supply roller 381 integral with the drive shaft 380 and the pressure cam 389 are rotated in the direction shown by the arrow, as shown in FIG. 29, the roller 387a is engaged by the smaller diameter portion of the pressure cam 389 to rise upwardly, with the result that the sheets P.sub.1 on the intermediate plate 383 are urged against the sheet supply roller 381 and are fed by the sheet supply roller 381.
When the projection of the drive ring 393 is engaged and locked by the pawl 391a again after one revolution thereof, the sheet supply roller 381 is stopped and at the same time the support arm 387 returns to the condition shown in FIG. 28, with the result that the engagement between the sheet P.sub.1 and the sheet supply roller 381 is released.
However, in the above-mentioned conventional sheet feeding apparatus, the following problems arose. (1). Between an outer diameter of the sheet supply roller 381 and a sheet path length l.sub.1 extending from the sheet supply roller 381 to the paired regist rollers 317, the physical regulation or limitation D.pi.&gt;l.sub.1 &gt;(D.pi.-.alpha.) arises (where, .alpha.=about 0.about.15; D=diameter of roller). That is to say, the sheet path length l.sub.1 is limited by the outer diameter of the sheet supply roller 381, or the outer diameter of the sheet supply roller 381 must be increased to maintain the adequate sheet path length l.sub.1, thereby making the apparatus bulky. (2) Whenever each sheet is supplied, the intermediate plate 383 must be lifted and lowered once via the support arm 387 through one revolution of the pressure cam 389. (3) Since the support arm 387 must be lifted, the profile of the pressure cam 389 has an abruptly inclined portion 389a, with the result that the impact between the intermediate plate 383 and the sheet supply roller 381 generates a noise. (4) Since the drive shaft 380 is integrally formed with the sheet supply roller 381, the paired regist rollers 317 are subjected to a considerable back tension. (5) Due to the above reasons (1)-(3), the sheet stacking ability is limited to some extent.